When two bodies, particularly of unlike materials, are brought together into intimate contact, a redistribution of electrons across the interface is likely to occur. An attractive force is established as equilibrium is achieved. Work must be done in opposition to these attractive forces if and when the bodies are separated. The energy so expended manifests itself as an increase in electrical tension or voltage between the respective surfaces, which become electrically charged with respect to each other. If a conductive path is available, the charges thus separated will reunite immediately. If no such path is available, as in the case of nonconductors, the potential increase with separation may reach values of several thousand volts. The generation of such electrical forces by contact is known as triboelectricity.
The charge on a charged object will be located on the exterior surface thereof, and these forces have a strong influence on nearby objects. If a neighboring object is a conductor it will experience a separation of charges by induction. Its repelled charge is free to give or receive electrons as the case may be; if another conductor is brought near, the transfer may occur through the agency of the spark, very often an energetic spark.
Triboelectrically generated charges may adversely affect or even electronically destroy a number of electronic circuits or solid state devices sensitive to sudden or stray electric charges or static electricity. Micro-circuit devices such as integrated circuit chips may be destroyed or weakened by electrostatic discharge prior to their incorporation into the electrical or electronic equipment for which they were designed. Damage from electrostatic discharge may make such devices prone to latent or catastrophic failure during use.
To prevent electrostatic breakdown, containers in which such devices are stored and transported have been provided with means for short circuiting the device terminals or pins. This short circuiting serves to prevent the accumulation of potentially damaging static charges on the device.
U.S. Pat. No. 4,171,049 discusses the utilization of a series of conductive slots or grooves in which solid state devices may be inserted and later dispensed to manufacturing equipment.
Other containers have been developed for portable use as for example in the device replacement market. These are typically small box like containers that house conductive sponge or foam sheets into which the device terminals are temporarily imbedded. An example is found in U.S. Pat. No. 4,333,565.
In addition to the requirements for inhibiting electrostatic charge build up and shielding solid state devices from electrical fields, a container useful for transporting such devices should also provide protection from mechanical shock and vibration. In addition, such a container should be of lightweight construction and convenient to use when gaining access to the equipment to be repaired. The container should also be adaptable to the storage of different sizes and shapes of solid state devices without change in the size and shape of the container itself. Reuseability, tamper security, and cost economy are also important considerations.
Many of the containers referred to above are designed for online infactory production use and do not meet many of the requirements desirable in a portable container. For example, many of the prior art containers are not adaptable to solid state devices of different shapes and sizes.
Another means well known in the art for providing protection of solid state devices is the use of carbon black or carbon powder in varying amounts in the material forming the container for the solid state devices.
U.S. Pat. No. 4,494,651 issued to Malcolm discloses a portable work station in which electrically conductive material, such as carbon black particles, aluminum particles, and metal filings, is blended with thermoplastic material to make an electrically conductive case.
Electrically conductive containers are often provided with an electrically conductive or antistatic foam pad which is used to line the bottom interior surface of the container. The foam pad has hygroscopic wetting or other antistatic agents incorporated therein or thereon.
The presence of the wetting agents, which attract moisture to the surface of the pad, substantially diminishes the possibility of static electricity being generated in the interior of the container as the result of friction between the solid state devices and the foam pads supporting the devices. Moreover, since the foam pad has been rendered electrically conductive and is in electrically conductive communication with the container, any static charge which may be internally generated will harmlessly float to the surface of the enclosure means.
A preferred material for the pad is known in the art as "pink poly". This material is a polyethylene, such as low density polyethylene, which has been impregnated with an organic liquid which can act as a hygroscopic wetting agent to attract and hold moisture onto the surface of the pad to render the pad electrically conductive. The pad may also be formed in an electrically conductive loaded plastic foam such as carbon or aluminum loaded polyurethane foam. This second material differs from the first material in that the electrical conductivity is present throughout the foam pad as opposed to the surface only of the pad. Still another type of conductive pad is a polyethylene foam which has been sprayed with a conductive carbon or similar conductive solution to affect a conductive surface layer.
These pads are typically preshaped to the configuration of the solid state device to accommodate the shape of the container, and are designed to direct static charges generated by frictional movement or other slight vibrations of the solid state device against the pad to the surface of the container. Typical hygroscopic wetting agents used in conjunction with the internal foam pads are well known to those skilled in the art and disclosed for example in U.S. Pat. No. 3,355,313 Eastes and Canadian Patent No. 810,595 also issued to Eastes.
One negative feature associated with the use of such internal pads, such as "pink poly" pads is that the anti-blocking or wetting agent used to provide the antistatic effect and to render the pad electrically conductive, may also deteriorate a solid state device.
It has also been found that at elevated temperatures volatiles can be produced in the pink polyethylene foam pad which can corrode a solid state device.
Of course, an additional consideration is the cost of installing these pads in each container.
It is, therefore, an object of the present invention to provide a means for protecting solid state devices within an electrically conductive container without the need for internal polyethylene foam pads or the like.